1. Field of the Invention
The present invention is related to a zeolite material having a nanocrystalline structure, and to a method for making same.
2. Background of the Art
Zeolites are porous crystalline materials characterized by submicroscopic channels of a particular size and/or configuration. Zeolites are typically composed of aluminosilicate, but have been made in a wide range of compositions. The channels, or pores are ordered and, as such, provide unique properties which make zeolites useful as catalysts or absorbents in industrial processes. For example, zeolites can be used for filtering out smaller molecules which become entrapped in the pores of the zeolite. Also, zeolites can function as shape selective catalysts which favor certain chemical conversions within the pores in accordance with the shape or size of the molecular reactants or products. Zeolites have also been useful for ion exchange, such as for water softening and selective recovery of heavy metals.
Synthetic zeolites are traditionally made from sources of silica and aluminum (silica and alumina xe2x80x9cnutrientsxe2x80x9d) that are reacted with each other, in the presence of materials that ensure highly alkaline conditions, such as water and OHxe2x88x92. Other zeolites can be borosilicates, ferrosilicates, and the like. Many of the crystallization steps are conducted in the presence of an inorganic directing agent, or an organic template, which induces a specific zeolite structure that cannot easily be formed in the absence of the directing agent or template. Many of the organic templates are quaternary ammonium salts, but can also be linear amines, alcohols, and the like. As a hydroxide, some is directing agents introduce hydroxyl ions into the reaction system; however, the alkalinity is usually dictated by the amount of sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. The reaction typically involves a liquid gel phase in which rearrangements and transitions occur, such that a redistribution occurs between the alumina and silica nutrients and molecular structures corresponding to specific zeolites are formed. Other metal oxides can also be included, such as titania-silica, boria-silica, etc. Some zeolites can only be made with organic templates. Other zeolites can only be made by means of an inorganic directing agent. Yet other zeolites can be made either by means of a hydrophilic (e.g., inorganic) directing agent or a hydrophobic (organic based) template.
Much of today""s hydrocarbon processing technology is based on zeolite catalysts. Various zeolite catalysts are known in the art and possess well-arranged pore systems with uniform pore sizes. The term xe2x80x9cmedium porexe2x80x9d as applied to zeolites usually refers to zeolite structures having a pore size of 4-6 angstrom units (xc3x85). xe2x80x9cLarge porexe2x80x9d zeolites include structures having a pore size of above 6 to about 12 xc3x85.
Because such hydrocarbon processing reactions are limited by mass-transfer (specifically, intraparticle diffusion), a catalyst with ideal pore size will facilitate transport of the reactants to active catalyst sites and transport of the products out of the catalyst, but still achieve the desired shape selective catalysis. Zeolite morphology, i.e., crystal size, is another parameter in diffusion limited reactions.
U.S. Pat. No. 6,004,527, which is herein incorporated by reference in its entirety, discloses a hydrothermal method for synthesizing large pore zeolites from an amorphous framework structure. The method produces a molecular sieve in-situ which is incorporated in the large pore framework morphology of the amorphous starting material. However, the method described therein requires a long period of time for the synthesis. It would be advantageous to have a convenient, less time-consuming method for producing a similar product.
A method is provided herein for making a molecular sieve. The method comprises the steps of: (a) providing a porous inorganic oxide; (b) impregnating the porous inorganic oxide with a liquid solution containing a micropore-forming directing agent, wherein the amount of liquid solution is no more than about 100% of the pore volume of the porous inorganic oxide, and the concentration of the directing agent in the liquid solution ranges from about 21% to about 60% by weight; and, (c) heating the impregnated porous inorganic oxide at an elevated synthesis temperature (i.e., above room temperature) for a duration of time sufficient to form a zeolite-containing product. The preferred product is a zeolite such as zeolite Y.
The method herein advantageously produces zeolite having a nanocrystalline (i.e., a crystal size of no more than about 100 nm) structure in a relative short synthesis time. While the porous inorganic oxide is usually amorphous, it can alternatively be partially or fully crystalline.